Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Subcutaneous cardiac stimulation devices are also known in the art. In these devices, the device enclosure may also be implanted beneath the skin of a patient. However, in these systems, the electrodes are not implanted within the heart. Rather, the electrodes may still be placed beneath the skin of the patient but external to the heart.
Subcutaneous cardiac devices are generally easier to implant. They are not generally relied on for providing long term pacing because the pacing efficiency of subcutaneous electrodes is low. In order to reliably pace, for example, stimulation pulse energies may be required which would rapidly deplete the battery of the device. Also of significance is the potential pain that may be caused by the required stimulation energies and electrode placement. However, subcutaneous cardiac stimulation devices may be advantageous for use in patients who do not require long term pacing, but who may have the potential to require sporadic cardiac stimulation therapy, such as for the abnormally high heart rate of an occasional tachyarrhythmia. Such conditions may be treated with anti-tachycardia pacing (ATP) to return the heart rate to a normal rate. Without such ATP, the heart rate may continue to accelerate into a more life threatening arrhythmia, such as ventricular fibrillation.
ATP is well known in the art. In such therapy, the heart is paced at a rate faster than the intrinsic rate. The heart beat is captured by the ATP and the pacing rate is gradually decreased to return the heart to a normal rate.
Even though subcutaneous cardiac stimulation devices are well suited for delivering ATP to a heart, the pain that such therapy may cause remains an issue. It is to this issue and the improved delivery of ATP with a subcutaneous cardiac stimulation device that the present invention more generally relates.